1. Field of the Invention
This invention relates to a process of treating sewage, particularly gas condensates from coal-gasifying plants and/or coal chemical plants, which sewage has a total content of at least 2 mval/l of the anions SO.sub.4.sup.--, SCN.sup.-, NO.sub.3 .sup.-, Cl.sup.- and F.sup.- and contains organic matter in an amount corresponding to a chemical oxygen demand (COD) of at least 1000 mg/l, wherein the sewage is passed thrugh a biological purification stage and a succeeding fine purification stage, strong anions are exchanged in an anion exchanger with hydrogen carbonate ions, and at least part of the treated water is added to the sewage before the biological purification stage.
Such process has been described in Laid-open German Application No. 31 09 848 and is further developed here.
In the conversion of coal and silimar fuels, e.g., by gasification, a highly soiled sewage becomes available, which contains components of the product gas produced by the gasification. The sewage which contains a condensate of the product gas is passed through a plurality of coarse purification stages to remove tars, oils, phenols, acid gases and ammonia. Such treatment has been described in Chemical Engineering Progress, volume 71, No. 6 (1975) on pages 99 to 104. In that treatment, large proportions of the phenols are removed, e.g., by liquid-liquid extraction. Ammonia can be removed in part by stripping with vapor. However, after such coarse purification, the sewage still contains various disturbing components, such as sulfate, thiocyanate, chlorine, fluorine and nitrate ions as well as free NH.sub.3 and ammonium ions and organic impurities so that the sewage must not be directly discarded into a receiving body of water and cannot be used further, e.g., as cooling water. For this reason the sewage is then purified biologically and subsequently subjected to a fine purification. Strong anions (SO.sub.4.sup.--, NO.sub.3.sup.-, Cl.sup.-, F.sup.-) are exchanged with hydrogen carbonate ions in an anion exchange stage, and at least part of the treated water is recycled to a point preceding the biological purification stage. In that process the treated water, which contains CO.sub.3 /HCO.sub.3 ions, effects a compensation of the cation deficiency and a stabilization of the pH value in the biological purification stage.
Alkali ions are supplied to the system with the sewage. The treatment in the biological purification zone preferably includes nitrification and denitrification and without the above-mentioned recycling of alkali ions would result in a cation deficiency. This is mainly due to the fact that nitrogen is taken up by the cell material of the microorganisms and that ammonia is converted to nitrite and nitrate by biological oxidation. This results in a strong decrease of the pH value. Whereas this could be opposed, e.g., by an addition of Ca(OH).sub.2 in order to avoid a disturbance of the biological processes, high concentrations of calcium would be disturbing in the further use as cooling water and would have to be prevented at a high expenditure. The addition of other foreign substances, such as sodium carbonate, is also expensive and they must be removed if the water is to be re-used. These disadvantages are avoided by the exchange of anions and by the recycling of the treated sewage. In this manner the pH value in the biological purification stage is maintained in the favorable range of about 6 to 8.
Owing to its circulation, the treated water which is recycled to the sewage is enriched with strong cations, namely, Na.sup.-, K.sup.-, Ca.sup.- and/or Mg.sup.- ions. The strong anions are removed at least in part in the anion exchanger. This may result in the system in an excessive surplus of free alkali, which disturbs the exchange of strong ions with (alkaline) hydrogen carbonate ions in the anion exchanger, which preferably contains a slightly basic anion exchange material. In that anion exchange, the existing alkali competes with the alkali which is to be formed.
For this reason it is an object of the invention to avoid in the process described first hereinbefore a disturbingly high surplus of free alkali in the water. This is accomplished in accordance with the invention in that water which has left the anion exchanger and has an alkalinity of at least 2 mval/l is treated in a cation exchange stage to remove at least part of the free alkali from the water before the latter is recycled to the sewage.
Because the free alkali must be removed only to a remainder of about 2 mval/l, it is sufficient in most cases to pass only a partial stream of the water through the cation exchanger.
If the treated water is to be used as cooling water in a cooling tower, it is desirable to remove the surplus free alkali before because a high alkalinity of cooling water gives rise to certain difficulties in the conditioning of the cooling water, e.g., when chlorine is used as a biocide. Moreover, the salt concentration in cooling water must be limited regardless of the kind of said salts.
The surplus free alkali is preferably removed by means of a slightly acid cation exchange material which is in the hydrogen form. Organic acids such as become available as sewage in the reaction water of hydrocarbon synthesis may be used to regenerate the cation exchange material. The regeneration eluate may also be supplied to the sewage before the biological purification and may be biologically decomposed. If mineral acids are used as regenerating liquor, the salts corresponding to said acids will be contained in the regeneration eluate so that said eluate must not be supplied to the system. If the cation exchange material is regenerated by means of organic acids, preferably fatty acids having a dissociation constant in excess of 10.sup.-5 are employed. The regeneration eluate is recycled to the biological processing stage, the alkali ions are not lost and are desirably available for effecting a neutralization in the biological purification stage. On the other hand, the water which has been treated by a cation exchange will have a correspondingly lower alkalinity, as is desirable for cooling water for use in a cooling tower. If alkali ions are to be removed from the system, the use of mineral acids for regenerating the cation exchange material and the discarding of the regeneration eluate is recommended.
An advantage afforded by the removal of surplus free alkali by a cation exchange resides in that any desired alkalinity in the effluent of the anion exchange stage can be adjusted so that the salt content will be reduced accordingly. Another advantage resides in that ammonium ions can also be removed in part in this manner.